Hydraulic seals are used, particularly in the aerospace industry, to prevent air from traversing the seal. Particularly, hydraulic seals are used to form a seal between two relatively rotating components. For example, a hydraulic seal may be used to seal an intershaft cavity between concentric co-rotating or counter-rotating shafts of a gas turbine engine.
FIG. 1 is a sectional view at top dead centre of a known intershaft hydraulic seal. The hydraulic seal is used to seal between concentrically arranged inner and outer shafts 102, 104 of a multi-spool engine.
The inner and outer shafts 102, 104 rotate at different speeds about a common axis and the hydraulic seal is used to seal a relatively high pressure region 106 from a relatively low pressure region 108.
The inner shaft 102 comprises an annular seal member or fin 110 which projects radially outward from the inner shaft 102 and extends around the entire circumference of the inner shaft 102.
The outer shaft 104 comprises an annular trough 112 which faces radially inward and extends around the entire circumference of the outer shaft 104.
The fin 110 of the inner shaft 102 is received within the trough 112 of the outer shaft 104.
The trough 112 is fed with hydraulic fluid (e.g. oil). The rotation of the outer shaft 104 creates sufficient centrifugal force to retain the hydraulic fluid within the trough 112. Accordingly, an annulus of fluid is formed in the trough 112.
The fin 110 is partially submerged in the annulus of fluid held by the trough 112. Consequently, a seal is formed and the difference in air pressure between the high pressure region 106 and the low pressure region 108 is compensated for by a difference in the fluid level on either side of the fin 110 (akin to a U-tube manometer).
In order for a seal to form, the outer shaft 104 must rotate at sufficient speed to create the annulus of fluid. Furthermore, a continuous feed of hydraulic fluid is required to fill the entire circumference of the trough 112 and to provide a circulation of fluid through the seal.
The sealing capability of an intershaft hydraulic seal at the point of seal formation is determined by the quantity of fluid within the hydraulic seal. The quantity of fluid within the seal is determined by a combination of the initial fluid feed and the retained volume of fluid within the hydraulic seal on shutdown (i.e. when all components have ceased rotation), around bottom dead centre.
Any direct fluid feed to the hydraulic seal from a pump can incur a delay, inherent in the complexity of the system. Therefore in isolation, the retained fluid volume can offer insufficient sealing capability during initial operation.
European Patent Application Number 09252342.2 discloses a hydraulic seal arrangement which is designed to ensure that sufficient fluid is retained within the seal on shutdown. The arrangement comprises an annular trough having a pair of seal exit weirs which allow excess fluid to exit the trough for scavenging. On passing over the seal exit weirs, the excess fluid enters an annular trap. The trap is provided with a pair of trap exit weirs which allow excess fluid to flow out of the trap.
On shutdown, and in the absence of centrifugal effects, the hydraulic fluid collects at the bottom of the trough. Accordingly, when the body of fluid passes one of the seal exit weirs, fluid will flow out of the trough and into the trap, and will do so until the fluid levels are equal on both sides of the seal exit weir.
Pockets are provided in the vicinity of the trap exit weirs to reduce the fluid level and to prevent the fluid from exiting the trap via the trap exit weirs. If the pockets were not provided, the trap would spill a small volume of fluid each time the trap exit weir approached bottom dead centre, which would be refilled from the trough as the seal exit weir approached bottom dead centre. Consequently, the trough could slowly drain until it retained insufficient oil to form an adequate seal.
Although this arrangement acts to maintain a minimum amount of hydraulic fluid in the trough, the actual volume of hydraulic fluid retained is limited by the geometry of the hydraulic seal. Consequently, there may be applications where this arrangement is unable to retain sufficient fluid.
Accordingly, the present invention seeks to provide a hydraulic seal arrangement which retains hydraulic fluid on shutdown in order to rapidly form a seal when operation recommences, and where the volume of retained fluid is independent of the geometry of the hydraulic seal.